Stack up assembly

ABSTRACT

A first printed circuit board is built including one or more openings configured to correspond to heat-generating devices attached to a second printed circuit board. The first and second printed circuit boards are aligned with each other and a heat sink, such that the heat sink is thermally coupled with heat-generating electronic devices on both the first and second printed circuit boards. Heat-generating devices are thermally coupled with a thermal pad on one or more of the printed circuit boards. The thermal pad is then thermally coupled with the heat sink. Optionally, the first and second printed circuit boards may be electrically coupled with each other through an electrical connector.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of application Ser. No.10/425,548 also entitled, “Stack Up Assembly,” filed on Apr. 28, 2003hereby incorporated herein by reference. application Ser. No. 10/425,548entitled, “Stack Up Assembly,” filed on Apr. 28, 2003 is acontinuation-in-part of application Ser. No. 10/425,491 also entitled,“Stack Up Assembly,” filed on Apr. 28, 2003, and also herebyincorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to the field of heatsinks and more specifically to the field of heat sinks configured toconduct heat from heat-generating devices on two or more differentprinted circuit boards.

BACKGROUND OF THE INVENTION

[0003] Modern electronics have benefited from the ability to fabricatedevices on a smaller and smaller scale. As the ability to shrink deviceshas improved, so has their performance. Unfortunately, this improvementin performance is accompanied by an increase in power as well as powerdensity in devices, resulting in large amounts of heat. In order tomaintain the reliability of these devices, the industry must find newmethods to remove this heat efficiently.

[0004] Many current systems include a plurality of printed circuitboards. These boards may each include a plurality of heat-generatingdevices requiring cooling to remain within their operating temperatures.Some commonly available current systems configure the printed circuitboards such that they are parallel with each other and then forceairflow across the printed circuit boards, thus cooling theheat-generating devices attached to the printed circuit boards. Theindividual heat-generating devices may include heat sinks to make moreefficient use of the heat transfer properties of the airflow. However,as devices shrink in size and heat generation increases, standardtechniques such as individual heat sinks and wide gaps between parallelprinted circuit boards are no longer sufficient to provide the compactsize required of many devices today.

[0005] Some printed circuit boards and their devices are configured toallow the use of a single heat sink across a plurality of individualheat-generating devices. This allows the use of larger heat sinks thatare more efficient and cheaper and easier to manufacture than aplurality of individual heat sinks. Often, two printed circuit boardscontain devices with functions that must be closely mated for optimalperformance. For example, a power module board is most effective when itis as close as possible to the printed circuit board including the ASICsor microprocessors to which the power module board is supplying power.This closeness reduces voltage drops along the, now shortened,interconnect between the power module and the ASICs or microprocessors.Typically, devices on both the power module board and the microprocessorprinted circuit board require heat sinks to efficiently dissipate theheat generated by the electronic devices on those boards. One techniqueinvolves placing the power module board and the printed circuit boardback-to-back with their heat sinks facing outwards from the two boards.However, this technique results in a system requiring two airflows overthe two sets of heat sinks for efficient cooling. This requirementcauses the overall volume of the completed device to increase, alongwith the cost of providing two airflows. Similarly, when a singleprinted circuit board is used and the power module is placed on theopposing side of the printed circuit board, two sets of heat sinks andtwo airflows are still required. Other configurations may place thepower module components on the same side of a single printed circuitboard with the other components, reducing the airflows required to one.However, this configuration may not allow the shortest possible powersupply connections to the ASICs, microprocessors, or other devices.

SUMMARY OF THE INVENTION

[0006] A first printed circuit board is built including one or moreopenings configured to correspond to heat-generating devices attached toa second printed circuit board. The first and second printed circuitboards are aligned with each other and a heat sink, such that the heatsink is thermally coupled with heat-generating devices on both the firstand second printed circuit boards. Within the scope of the presentinvention the heat sink may be a heat spreader, cold plate,refrigeration (evaporative cooling) plate, heat pipe or any other deviceconfigured to remove heat from the heat-generating devices.Heat-generating devices are thermally coupled with a thermal pad on oneor more of the printed circuit boards. Optionally, the first and secondprinted circuit boards may be electrically coupled with each otherthrough an electrical connector. Also optionally, heat-generatingdevices may be mechanically and electrically coupled with the secondprinted circuit board through interposers configured (upon assembly) toraise the heat-generating electronic devices through the openings in thefirst printed circuit board such that the upper surfaces of theheat-generating devices of the first and second circuit boards aresubstantially co-planar. Optionally, more than two printed circuitboards with any combination of openings, heat-generating devices andthermal pads may be used within the scope of the present invention.

[0007] Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1A is a top view of an example embodiment of a first printedcircuit board including heat-generating devices according to the presentinvention.

[0009]FIG. 1B is a cross-sectional view of the example embodiment of afirst printed circuit board from FIG. 1A along section line A-A.

[0010]FIG. 2A is a top view of an example embodiment of a second printedcircuit board including heat-generating devices according to the presentinvention.

[0011]FIG. 2B is a cross-sectional view of the example embodiment of asecond printed circuit board from FIG. 2A along section line B-B.

[0012]FIG. 3A is a cross-sectional view of an example stack up assemblybefore assembly of the example embodiments of first and second printedcircuit boards from FIGS. 1 and 2 along with an example embodiment of aheat sink according to the present invention.

[0013]FIG. 3B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiments of first and secondprinted circuit boards from FIGS. 1 and 2 along with an exampleembodiment of a heat sink according to the present invention.

[0014]FIG. 4A is a top view of an example embodiment of a second printedcircuit board including heat-generating devices according to the presentinvention.

[0015]FIG. 4B is a cross-sectional view of the example embodiment of asecond printed circuit board from FIG. 4A along section line C-C.

[0016]FIG. 5A is a cross-sectional view of an example stack up assemblybefore assembly of the example embodiments of first and second printedcircuit boards from FIGS. 1 and 4 along with an example embodiment of aheat sink according to the present invention.

[0017]FIG. 5B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiments of first and secondprinted circuit boards from FIGS. 1 and 4 along with an exampleembodiment of a heat sink according to the present invention.

[0018]FIG. 6 is a flow chart of an example method for the constructionof a stack up including first and second printed circuit boards cooledby a single heat sink according to the present invention.

[0019]FIG. 7A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present inventionincluding a total of three printed circuit boards and a heat sink.

[0020]FIG. 7B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention from FIG. 7A.

[0021]FIG. 8A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present inventionincluding a printed circuit board with heat-generating devices on bothsides.

[0022]FIG. 8B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention from FIG. 8A.

[0023]FIG. 9A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present inventionincluding a total of five printed circuit boards and a heat sink.

[0024]FIG. 9B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention from FIG. 9A.

[0025]FIG. 10A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present invention asshown in FIG. 5A along with gap-filling thermal interfaces between theheat-generating devices and the heat sink.

[0026]FIG. 10B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention as shown in FIG. 10A.

DETAILED DESCRIPTION

[0027]FIG. 1A is a top view of an example embodiment of a first printedcircuit board including heat-generating devices according to the presentinvention. In this example embodiment of the present invention a firstprinted circuit board 100 including a first opening 102, a secondopening 104, a third opening 106, and a fourth opening 108 is provided.Other embodiments of the present inventions may include any number ofopenings as needed for a particular implementation of the presentinvention. Also included on this first printed circuit board 100 are anumber of first heat-generating devices 110. The terminology “firstheat-generating devices” is used to distinguish these heat-generatingdevices on the first printed circuit board from those present on thesecond printed circuit board discussed below. As shown in FIGS. 5A and5B, an upper surface of the first heat-generating devices may besubstantially coplanar with an upper surface of the heat-generatingdevices on the second printed circuit board. While this exampleembodiment of the present invention included five first heat-generatingdevices 110, other embodiments may include any number of firstheat-generating devices 110 as needed for a particular implementation ofthe present invention. These first heat-generating devices 110 mayinclude electronic power circuits, application specific integratedcircuits (ASICs), microprocessors, discrete electronic devices such asfield effect transistors (FETs), other types of transistors, or otherheat-generating devices as needed for a particular implementation of thepresent invention. In some embodiments of the present invention thisfirst printed circuit board 100 may be a power module circuit board, avoltage regulation module (VRM) circuit board, or any other type ofdevice as needed for a particular implementation of the presentinvention.

[0028]FIG. 1B is a cross-sectional view of the example embodiment of afirst printed circuit board from FIG. 1A along section line A-A. In thisexample embodiment of the present invention, the first printed circuitboard 100 is shown with a first opening 102, and a second opening 104.Also shown in this cross-sectional view is one of the upperheat-generating electronic devices 110 from FIG. 1A.

[0029]FIG. 2A is a top view of an example embodiment of a second printedcircuit board including heat-generating electronic devices according tothe present invention. In this example embodiment of the presentinvention a second printed circuit board 200 is provided including asecond heat-generating device 202, a third heat-generating device 204, afourth heat-generating device 206, and a fifth heat-generating device208. Other embodiments of the present invention may include any numberof heat-generating devices as needed for a particular implementation ofthe invention. These heat-generating devices 202, 204, 206, and 208 mayinclude electronic power circuits, application specific integratedcircuits (ASICs), microprocessors, discrete electronic devices such asfield effect transistors (FETs), other types of transistors, or otherheat-generating electronic devices as needed for a particularimplementation of the present invention. Also included on this secondprinted circuit board 200 are a number of other devices 210 that may ormay not generate heat, along with a plurality of discrete devices 212,(such as resistors, capacitors, transistors, and diodes, for example)that also may or may not generate heat. Those of skill in the art willrecognize that any of the printed circuit boards may include discretedevices 212, or other heat-generating devices that are not directlycoupled with the heat sink.

[0030] Optionally, thermal pads 214 may be placed on the printed circuitboard that are thermally coupled to the discrete devices 212 and thesethermal pads 214 may then be contacted by a heat sink to remove heatfrom the discrete devices 212. Optionally, one or more of theheat-generating devices 202, 204, 206, and 208 may be thermally coupledto the thermal pads 214. In some embodiments of the present inventionthese pads 214 may be standard copper printed circuit board pads. Thisoptional embodiment of the present invention is shown and described inFIGS. 8A and 8B.

[0031]FIG. 2B is a cross-sectional view of the example embodiment of asecond printed circuit board from FIG. 2A along section line B-B. Inthis example embodiment of the present invention, the second printedcircuit board 200 is shown with a second heat-generating device 202, athird heat-generating device 204, and two discrete electronic devices212.

[0032]FIG. 3A is a cross-sectional view of an example stack up assemblybefore assembly of the example embodiments of first and second printedcircuit boards from FIGS. 1 and 2 along with an example embodiment of aheat sink according to the present invention. This example embodiment ofa stack up according to the present invention includes the first printedcircuit board 100 from FIG. 1, the second printed circuit board 200 fromFIG. 2, along with an example embodiment of a heat sink 300 according tothe present invention. Those of skill in the art will recognize that awide variety of thermal devices may be used as a heat sink 300. While astandard finned heat sink 300 is shown in FIGS. 3A and 3B, other exampleembodiments of the present invention may use heat spreaders, coldplates, refrigeration (evaporative cooling) plates, heat pipes, or otherthermal devices in place of the finned heat sink shown in these figures.This cross-sectional view of an example stack up shows the first printedcircuit board 100 from FIG. 1B and the second printed circuit board 200from FIG. 2B. In this example embodiment of the present invention, thefirst printed circuit board 100 is shown with a first opening 102, and asecond opening 104. Also shown in this cross-sectional view is one ofthe first heat-generating devices 110 from FIG. 1A. In this exampleembodiment of the present invention, the second printed circuit board200 is shown with a second heat-generating device 202, a thirdheat-generating device 204, and two discrete devices 212. Note that theheat sink 300 includes a first protrusion 302, and a second protrusion304 configured to pass through the first opening 102 and the secondopening 104 of the first printed circuit board 100 and make contact withthe second heat-generating device 202 and the third heat-generatingdevice 204 on the second printed circuit board 200. Those of skill inthe art will recognize that there is no requirement that the bottomsurfaces of the first protrusion 302 and the second protrusion 304 beco-planar. Note that in some embodiments of the present invention, theheat sink 300 may be a thermal plate, a vapor plate, a heat pipe, or anyother thermal device capable of removing heat from the heat-generatingdevices on the first and second printed circuit boards.

[0033]FIG. 3B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiments of first and secondprinted circuit boards from FIGS. 1 and 2 along with an exampleembodiment of a heat sink according to the present invention. After theexample stack up shown in FIG. 3A is assembled, the first printedcircuit board 100 is mechanically and electrically coupled with thesecond printed circuit board 200 through one or more electricalconnectors 306. These electrical connectors 306 may be configured to setthe distance between the first and second printed circuit boards 100,and 200 such that the heat sink 300 makes thermal contact with the firstheat-generating devices 110 on the first printed circuit board 100 alongwith the heat-generating devices 202, and 204 on the second printedcircuit board 200. The discrete devices 212 attached to the secondprinted circuit board 200 in this example embodiment of the presentinvention are not thermally coupled to the heat sink. Those of skill inthe art will recognize that these discrete devices 212 may not requirecooling through the heat sink 300 if their heat output is low. Also,there may be some cooling of these devices 212 by air flowing betweenthe first and second printed circuit boards 100, and 200. While thisexample stack up of the present invention shows two openings 102, and104 in the first printed circuit board 100 and two heat-generatingdevices 202, and 204 attached to the second printed circuit board 200,those of skill in the art will recognize that any number of openings inthe first printed circuit board 100 may be used to provide heat sinkaccess to any number of heat generating devices on the second printedcircuit board 200.

[0034]FIG. 4A is a top view of an example embodiment of a second printedcircuit board including heat-generating devices according to the presentinvention. This example embodiment of the present invention is similarto that shown in FIGS. 2A and 2B. However, in this example embodiment ofthe present invention, the heat-generating devices 402, 404, 406, and408 are packaged in pin grid array (PGA) packages and supported byinterposers 414 attached to the second printed circuit board 400. Inthis example embodiment of the present invention a second printedcircuit board 400 is provided including a second heat-generating device402, a third heat-generating device 404, a fourth heat-generating device406, and a fifth heat-generating device 408. These heat-generatingdevices 402, 404, 406, and 408 are mechanically and electrically coupledwith the second printed circuit board 400 through interposers 414 thatare shown in FIG. 4B. Other embodiments of the present invention mayinclude any number of heat-generating devices as needed for a particularimplementation of the invention. These heat-generating devices 402, 404,406, and 408 may include electronic power circuits, application specificintegrated circuits (ASICs), microprocessors, discrete electronicdevices such as field effect transistors (FETs), other types oftransistors, or other heat-generating devices as needed for a particularimplementation of the present invention. Also included on this secondprinted circuit board 400 are a number of other devices 410 that may ormay not generate heat, along with a plurality of discrete devices 412,(such as resistors, capacitors, transistors, and diodes, for example)that also may or may not generate heat.

[0035]FIG. 4B is a cross-sectional view of the example embodiment of asecond printed circuit board from FIG. 4A along section line C-C. Inthis example embodiment of the present invention, the second printedcircuit board 400 is shown with a second heat-generating device 402, athird heat-generating device 404, and two discrete devices 412. Thesecond and third heat-generating devices 402, and 404 are mechanicallyand electrically coupled to the second printed circuit board 400 throughinterposers 414. Note that in some example embodiments of the presentinvention the interposers 414 may also include a socket configured toallow insertion and removal of the heat-generating devices 402, and 404.Interposers 414 are often used to allow non-permanent electrical andmechanical coupling of electronic devices to a printed circuit board.

[0036]FIG. 5A is a cross-sectional view of an example stack up assemblybefore assembly of the example embodiments of first and second printedcircuit boards from FIGS. 1 and 4 along with an example embodiment of aheat sink according to the present invention. This example embodiment ofa stack up according to the present invention includes the first printedcircuit board 100 from FIG. 1, the second printed circuit board 400 fromFIG. 4, along with an example embodiment of a heat sink 500 according tothe present invention. In this example embodiment of the presentinvention, the first printed circuit board 100 is shown with a firstopening 102, and a second opening 104. Also shown in thiscross-sectional view is one of the first heat-generating devices 110from FIG. 1A. In this example embodiment of the present invention, thesecond printed circuit board 400 is shown with a second heat-generatingdevice 402, a third heat-generating device 404, two discrete devices412, and two interposers 414 supporting the second and thirdheat-generating devices 402, and 404. Note that the heat sink 500includes a substantially flat bottom surface unlike the heat sink 300shown in FIG. 3.

[0037]FIG. 5B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiments of first and secondprinted circuit boards from FIGS. 1 and 4 along with an exampleembodiment of a heat sink according to the present invention. After theexample stack up shown in FIG. 5A is assembled, the first printedcircuit board 100 is mechanically and electrically coupled with thesecond printed circuit board 400 through one or more electricalconnectors 502. These electrical connectors 502 may be configured to setthe distance between the first and second printed circuit boards 100,and 400 such that the heat sink 500 makes thermal contact with the firstheat-generating devices 110 on the first printed circuit board 100 alongwith the heat-generating devices 402, and 404 on the second printedcircuit board 400. Note that the interposers 414 mechanically andelectrically coupling the heat-generating electronic devices 402, and404 to the second printed circuit board 400 are configured to positionthe heat-generating devices such that their top surfaces aresubstantially co-planar with each other and the heat-generating devices110 attached to the first printed circuit board 100. This allows the useof a single heat sink 500 with a substantially planar bottom surface tocontact all of the heat-generating devices 110, 402, and 404 on thefirst and second printed circuit boards 100, and 400 that the designerdesires to be thermally coupled to the heat sink 500. The discretedevices 412 attached to the second printed circuit board 400 in thisexample embodiment of the present invention are not thermally coupled tothe heat sink. Those of skill in the art will recognize that thesediscrete devices 412 may not require cooling through the heat sink 500if their heat output is low. Also, there may be some cooling of thesedevices 412 by air flowing between the first and second printed circuitboards 100, and 400. While this example stack up of the presentinvention shows two openings 102, and 104 in the first printed circuitboard 100 and two heat-generating devices 402, and 404 attached to thesecond printed circuit board 400, those of skill in the art willrecognize that any number of openings in the first printed circuit board100 may be used to provide heat sink access to any number ofheat-generating devices on the second printed circuit board 400.

[0038]FIG. 6 is a flow chart of an example method for the constructionof a stack up including first and second printed circuit boards cooledby a single heat sink according to the present invention. In a step 602,a first printed circuit board including a first heat-generating deviceand having a first opening is provided. In a step 604, a second printedcircuit board including a thermal pad is provided. In a step 606, asecond heat-generating device is electrically and mechanically coupledto the second printed circuit board. In a step 608, the secondheat-generating device is thermally coupled to the thermal pad. In astep 610, the first and second printed circuit boards are mechanicallycoupled. In an optional step 612, the first and second printed circuitboards are electrically coupled through an electrical connector. In astep 614, a heat sink having a first protrusion is provided. In a step616, the heat sink is mechanically coupled to the first and secondprinted circuit boards. In a step 618, the heat sink is thermallycoupled to the first heat-generating device and the thermal pad.

[0039]FIG. 7A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present inventionincluding a total of three printed circuit boards and a heat sink. Thisexample embodiment of a stack up according to the present inventionincludes a first printed circuit board 706 similar to that shown in FIG.1, a second printed circuit board 716 similar to that shown in FIG. 1,and a third printed circuit board 726 similar to that shown in FIG. 4,along with an example embodiment of a heat sink 700 according to thepresent invention. In this example embodiment of the present invention,the first printed circuit board 706 is shown with a first opening 708, asecond opening 710, and a third opening 712. Also shown in thiscross-sectional view is a first heat-generating device 714. In thisexample embodiment of the present invention, the second printed circuitboard 716 is shown with a second heat-generating device 722, and twodiscrete devices 724. The third printed circuit board 726, includes athird heat-generating device 728, a fourth heat-generating device 730,and some discrete devices 732. The heat sink 700 includes a firstprotrusion 702, a second protrusion 704, and a third protrusion 705.Note that the third protrusion 705 is shorter than the first and secondprotrusions 702, and 704 allowing the third protrusion 705 to makecontact with an upper surface of the second heat-generating device 722on the second printed circuit board 716 after assembly. The firstprotrusion 702 is configured to contact an upper surface of the thirdheat-generating device 728 on the third printed circuit board 726 afterassembly. The second protrusion 704 is configured to contact an uppersurface of the fourth heat-generating device 730 on the third printedcircuit board 726 after assembly.

[0040]FIG. 7B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention from FIG. 7A. After the example stack up shown in FIG. 7A isassembled, the first printed circuit board 706 is mechanically andelectrically coupled with the second printed circuit board 716 throughone or more electrical connectors 734, and the second printed circuitboard 716 is mechanically and electrically coupled with the thirdprinted circuit board 726 through one or more electrical connectors 736.These electrical connectors 734, and 736 may be configured to set thedistance between the printed circuit boards 706, 716, and 726 such thatthe heat sink 700 makes thermal contact with the first heat-generatingdevice 714 on the first printed circuit board 706, the secondheat-generating device 722 on the second printed circuit board 716,along with the heat-generating devices 728, and 730 on the third printedcircuit board 726. The discrete devices 732 attached to the thirdprinted circuit board 726 in this example embodiment of the presentinvention are not thermally coupled to the heat sink. Those of skill inthe art will recognize that these discrete devices 732 may not requirecooling through the heat sink 700 if their heat output is low. Also,there may be some cooling of these devices 732 by air flowing betweenthe second and third printed circuit boards 716, and 726. Otherembodiments of the present invention may thermally couple one or more ofthe discrete devices 732 through the third and fourth heat-generatingdevices 728, and 730 to the heat sink 700. While this example stack upof the present invention shows three openings 708, 710, and 712 in thefirst printed circuit board 706 and two heat-generating devices 728, and730 attached to the third printed circuit board 726, those of skill inthe art will recognize that any number of openings in the first printedcircuit board 706 may be used to provide heat sink access to any numberof heat generating devices on the second printed circuit board 716, andthe third printed circuit board 726. Those of skill in the art willrecognize that any number of printed circuit boards may be stacked upwithin the scope of the present invention.

[0041]FIG. 8A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present inventionincluding a printed circuit board with heat-generating devices on bothsides. This example embodiment of a stack up according to the presentinvention includes the first printed circuit board 100 from FIG. 1, ansecond printed circuit board 806 including heat-generating devices onboth sides of the PC board 806, along with an example embodiment of aheat sink 800 according to the present invention. Those of skill in theart will recognize that a wide variety of thermal devices may be used asa heat sink 800. While a standard finned heat sink 800 is shown in FIGS.8A and 8B, other example embodiments of the present invention may useheat spreaders, cold plates, refrigeration (evaporative cooling) plates,heat pipes, or other thermal devices in place of the finned heat sinkshown in these figures. In this example embodiment of the presentinvention, the first printed circuit board 100 is shown with a firstopening 102, and a second opening 104. Also shown in thiscross-sectional view is one of the first heat-generating devices 110from FIG. 1A. In this example embodiment of the present invention, asecond printed circuit board 806 is shown with a second heat-generatingdevice 808, a third heat-generating device 810, a fourth heat-generatingdevice 816, and a fifth heat-generating device 818. The third, forth andfifth heat-generating devices 810, 816, and 818 are thermally coupledwith a thermal pad 814 through a thermal trace 812 within the secondprinted circuit board 806. In the example embodiment of the presentinvention shown in FIGS. 8A and 8B, the thermal pad is an area of copperprinted circuit board that is thermally connected to the heat generatingdevices through copper traces on and within the second printed circuitboard 806. Some embodiments of the present invention may use one or moreground planes within the second printed circuit board 806 as a thermaltrace 812 coupled with a thermal pad 814 comprised of an area of copperon the surface of the second printed circuit board 806 coupled to theground planes. Other embodiments of the present invention may use othermaterials for the thermal pad 814, and other methods of thermallycoupling the heat-generating devices with the thermal pad 814 within thescope of the present invention. Note that the heat sink 800 includes afirst protrusion 802, and a second protrusion 804 configured to passthrough the first opening 102 and the second opening 104 of the firstprinted circuit board 100 and make contact with the secondheat-generating device 808 and the thermal pad 814 on the second printedcircuit board 806. Those of skill in the art will recognize that thereis no requirement that the bottom surfaces of the first protrusion 802and the second protrusion 804 are co-planar, and they are not co-planarin this example embodiment of the present invention. Note that in someembodiments of the present invention, the heat sink 800 may be a thermalplate, a vapor plate, a heat pipe, or any other thermal device capableof removing heat from the heat-generating devices on the first andsecond printed circuit boards.

[0042]FIG. 8B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention from FIG. 8A. After the example stack up shown in FIG. 8A isassembled, the first printed circuit board 100 is mechanically andelectrically coupled with the second printed circuit board 806 throughone or more electrical connectors 820. These electrical connectors 820may be configured to set the distance between the first and secondprinted circuit boards 100, and 806 such that the heat sink 800 makesthermal contact with the first heat-generating devices 110 on the firstprinted circuit board 100 along with the heat-generating device 808, andthermal pad 814 on the second printed circuit board 806.

[0043]FIG. 9A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present inventionincluding a total of five printed circuit boards and a heat sink 900.This example embodiment of a stack up according to the present inventionincludes a first printed circuit board 908, a second printed circuitboard 914, a third printed circuit board 920, a fourth printed circuitboard 928, a fifth printed circuit board 932, along with an exampleembodiment of a heat sink 900 according to the present invention. Inthis example embodiment of the present invention, the first printedcircuit board 908 is shown with a first opening 910, and a firstheat-generating device 912. In this example embodiment of the presentinvention, the second printed circuit board 914 is shown with a secondopening 916, and a third opening 918. The third printed circuit board920, includes a discrete device 924, and a fourth opening 922. Thefourth printed circuit board 928 includes a second heat-generatingdevice 930. The fifth printed circuit board 932, includes a thirdheat-generating device 934, a fourth heat-generating device 938, and aplurality of discrete devices 936. The heat sink 900 includes a firstprotrusion 902, a second protrusion 904, and a third protrusion 906.Note that the third protrusion 906 is shorter than the first and secondprotrusions 902, and 904 allowing the third protrusion 906 to makecontact with an upper surface of the second heat-generating device 930on the forth printed circuit board 928 after assembly. The firstprotrusion 902 is configured to contact an upper surface of the thirdheat-generating device 934 on the fifth printed circuit board 932 afterassembly. The second protrusion 904 is configured to contact an uppersurface of the fourth heat-generating device 938 on the fifth printedcircuit board 932 after assembly.

[0044]FIG. 9B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention from FIG. 9A. After the example stack up shown in FIG. 9A isassembled, the first printed circuit board 908 is mechanically andelectrically coupled with the third printed circuit board 920 throughone or more electrical connectors 940. The second printed circuit board914 is mechanically and electrically coupled with the third printedcircuit board 920 through one or more electrical connectors 942, and isalso mechanically and electrically coupled with the fourth printedcircuit board 928 through one or more electrical connectors 944. Thethird printed circuit board 920 is mechanically and electrically coupledwith the fifth printed circuit board 932 through one or more electricalconnectors 946. The fourth printed circuit board 928 is mechanically andelectrically coupled with the fifth printed circuit board 932 throughone or more electrical connectors 948. The electrical connectors 940,942, 944, 946, and 948 may be configured to set the distance between theprinted circuit boards 908, 914, 920, 928, and 932 such that the heatsink 900 makes thermal contact with the first heat-generating device 912on the first printed circuit board 908, the second heat-generatingdevice 930 on the fourth printed circuit board 928, along with theheat-generating devices 934, and 938 on the fifth printed circuit board932. The discrete devices 936 attached to the fifth printed circuitboard 932 in this example embodiment of the present invention are notthermally coupled to the heat sink. Those of skill in the art willrecognize that these discrete devices 936 may not require coolingthrough the heat sink 900 if their heat output is low. Also, there maybe some cooling of these devices 936 by air flowing between the third,fourth, and fifth printed circuit boards 920, 928, and 932. Otherembodiments of the present invention may thermally couple one or more ofthe discrete devices 936 through the third and fourth heat-generatingdevices 934, and 938 to the heat sink 900. While this example stack upof the present invention shows one opening 910 in the first printedcircuit board 908, two openings 916, and 918 in the second printedcircuit board 914, two openings 922, and 926 in the third printedcircuit board 920, and two heat-generating devices 934, and 938 attachedto the fifth printed circuit board 932, those of skill in the art willrecognize that any combination of openings and heat generating devicesmay be used within the scope of the present invention. Those of skill inthe art will recognize that any number of printed circuit boards may bestacked up within the scope of the present invention.

[0045]FIG. 10A is a cross-sectional view of an example stack up assemblybefore assembly of an example embodiment of the present invention asshown in FIG. 5A along with gap-filling thermal interfaces between theheat-generating devices and the heat sink. This example embodiment of astack up according to the present invention includes the first printedcircuit board 100 from FIG. 1, a second printed circuit board 1006similar to the printed circuit board shown in FIG. 4, along with anexample embodiment of a heat sink 1000 according to the presentinvention. Also, the stack up includes a middle frame 1002 and a lowerframe 1004. The heat sink 1000, middle frame 1002, and the lower frame1004 may be mechanically coupled to each other and the two printedcircuit boards 100, and 1006 to provide for mechanical stability,electrical coupling, and thermal coupling of the devices within themodule. Also, if the heat sink 1000, middle frame 1002, and lower frame1004 are metal, the combination acts as an electromagnetic interference(EMI) shield protecting electronic devices within the module from EMIexisting outside the module and also to prevent EMI generated within themodule from leaving the module. Thus, the heat sink 1000, middle frame1002, and lower frame 1004 may act as a faraday cage. In this exampleembodiment of the present invention, the first printed circuit board 100is shown with a first opening 102, and a second opening 104. Also shownin this cross-sectional view is one of the first heat-generating devices110 from FIG. 1A. In this example embodiment of the present invention,the second printed circuit board 1006 is shown with a secondheat-generating device 1008, a third heat-generating device 1010, andtwo interposers 1012 supporting the second and third heat-generatingdevices 1008, and 1010. Also included in this example embodiment aregap-filling thermal interfaces 1014 on top of the heat-generatingdevices 1008, and 1010. Upon assembly these gap-filling thermalinterfaces 1014 will provide thermal contact between the heat-generatingdevices 1008, and 1010 and the heat sink 1000. These gap-filling thermalinterfaces 1014 may be thermal paste, thermal pads, elastomeric thermalmaterial, or any other thermally conducting material suitable to conformto the upper surface of the heat-generating devices 1008, and 1010, andthe lower surface of the heat sink 1000, thus providing for greaterthermal coupling between the heat-generating devices 1008, and 1010 andthe heat sink 1000 than would be provided without the gap-fillingthermal interfaces 1014. While this example embodiment shows thegap-filling thermal interfaces 1014 positioned on top of theheat-generating devices 1008, and 1010 prior to assembly, those of skillin the art will recognize that the gap-filling thermal interfaces 1014could also be positioned on the lower surface of the heat sink 1000prior to assembly within the scope of the present invention.

[0046]FIG. 10B is a cross-sectional view of an example stack up assemblyafter complete assembly of the example embodiment of the presentinvention as shown in FIG. 10A. After the example stack up shown in FIG.10A is assembled, the first printed circuit board 100 is mechanicallycoupled with the second printed circuit board 1006 through the middleframe 1002. This middle frame 1002 may act as a positioning deviceconfigured to set the distance between the first and second printedcircuit boards 100, and 1006 such that the heat sink 1000 makes thermalcontact with the first heat-generating devices 110 on the first printedcircuit board 100 along with the heat-generating devices 1008, and 1010on the second printed circuit board 1006. Note that the interposers 1012mechanically and electrically coupling the heat-generating electronicdevices 1008, and 1010 to the second printed circuit board 1006 areconfigured to position the heat-generating devices such that their topsurfaces, including any gap-filling thermal interfaces 1014, aresubstantially co-planar with each other and the heat-generating devices110 attached to the first printed circuit board 100. This allows the useof a single heat sink 1000 with a substantially planar bottom surface tocontact all of the heat-generating devices 110, 1008, and 1010 on thefirst and second printed circuit boards 100, and 1006 that the designerdesires to be thermally coupled to the heat sink 1000. While thisexample stack up of the present invention shows two openings 102, and104 in the first printed circuit board 100 and two heat-generatingdevices 1008, and 1010 attached to the second printed circuit board1006, those of skill in the art will recognize that any number ofopenings in the first printed circuit board 100 may be used to provideheat sink access to any number of heat-generating devices on the secondprinted circuit board 1006.

[0047] The foregoing description of the present invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and other modifications and variations may be possible inlight of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical application to thereby enable others skilled in the art tobest utilize the invention in various embodiments and variousmodifications as are suited to the particular use contemplated. It isintended that the appended claims be construed to include otheralternative embodiments of the invention except insofar as limited bythe prior art.

What is claimed is:
 1. An assembly, comprising: a first printed circuitboard having a first opening, and including a first heat-generatingdevice; a thermal pad on a surface of said first printed circuit board,thermally coupled with said first heat generating device; a secondprinted circuit board mechanically coupled to said first printed circuitboard; a second heat-generating device; a first interposer configured tomechanically and electrically couple said second heat-generating deviceto said second printed circuit board, wherein said first interposer issubstantially aligned with said first opening in said first printedcircuit board; and a heat sink mechanically coupled with said first andsecond printed circuit boards, wherein said heat sink is thermallycoupled with said second heat-generating device, and said thermal pad.2. The assembly of claim 1, wherein said first interposer is ofsufficient height such that said second heat-generating device extendsthrough said first opening in said first printed circuit board, and anupper surface of said second heat-generating device is substantiallycoplanar with an upper surface of said first heat-generating device onsaid first printed circuit board.
 3. The assembly of claim 1, whereinsaid first heat-generating device is an ASIC.
 4. The assembly of claim1, wherein said first heat-generating device is a microprocessor.
 5. Theassembly of claim 1, wherein said first heat-generating device is a FET.6. The assembly of claim 1, wherein said second heat-generating deviceis an ASIC.
 7. The assembly of claim 1, wherein said secondheat-generating device is a microprocessor.
 8. The assembly of claim 1,wherein said second heat-generating device is a FET.
 9. The assembly ofclaim 1, wherein said first printed circuit board including a firstheat-generating device is a power module.
 10. The assembly of claim 1,wherein said second printed circuit board including a secondheat-generating device is a power module.
 11. The assembly of claim 1,wherein said first printed circuit board is a voltage regulation module(VRM) circuit board.
 12. The assembly of claim 1, wherein said secondprinted circuit board is a voltage regulation module (VRM) circuitboard.
 13. The assembly of claim 1, further comprising: an electricalconnector configured to electrically couple said first printed circuitboard to said second printed circuit board.
 14. A method for theconstruction of an assembly, comprising the steps of: a) providing afirst printed circuit board including a first heat-generating device andhaving a first opening; b) providing a second printed circuit board; c)creating a thermal pad on a surface of the first printed circuit board;d) mechanically and electrically coupling a second heat-generatingdevice to the second printed circuit board; e) thermally coupling thefirst heat-generating device to the thermal pad; f) providing a heatsink having a first protrusion configured to extend through the firstopening in the first printed circuit board and make thermal contact withthe second heat-generating device on the second printed circuit board;g) mechanically coupling the first printed circuit board with the secondprinted circuit board such that the second heat-generating device on thesecond printed circuit board is substantially aligned under the firstopening in the first printed circuit board; and h) mechanically couplingthe heat sink to the first and second printed circuit boards such thatthe first protrusion of the heat sink extends through the first openingin the first printed circuit board and makes thermal contact with thesecond heat-generating device on the second printed circuit board, andthe heat sink makes thermal contact with the thermal pad on the firstprinted circuit board.
 15. The method of claim 14, wherein the firstheat-generating device is an ASIC.
 16. The method of claim 14, whereinthe first heat-generating device is a microprocessor.
 17. The method ofclaim 14, wherein the first heat-generating device is a FET.
 18. Themethod of claim 14, wherein the second heat-generating device is anASIC.
 19. The method of claim 14, wherein the second heat-generatingdevice is a microprocessor.
 20. The method of claim 14, wherein thesecond heat-generating device is a FET.
 21. The method of claim 14,wherein the first printed circuit board including a firstheat-generating device is a power module.
 22. The method of claim 14,wherein the second printed circuit board including a secondheat-generating device is a power module.
 23. The method of claim 14,wherein the first printed circuit board is a voltage regulation module(VRM) circuit board.
 24. The method of claim 14, wherein the secondprinted circuit board is a voltage regulation module (VRM) circuitboard.
 25. The method of claim 14, further comprising the step of: i)electrically coupling the first printed circuit board to the secondprinted circuit board through an electrical connector.